1. Field of the Invention
This invention relates generally to handheld tweezer-like tools which, via conductive heat transfer, reflow solder and effect the installation and removal of electronic components with respect to a substrate. "Electronic components" or simply "components", as it is used herein, include any device, electronic, active, inactive or otherwise, which is mounted on, through or otherwise attached to a substrate, either mechanically, by solder, adhesive or any combination thereof. Substrates, as the term is used herein, encompasses any material to which components may be mounted. These materials include (but are not limited to) fiberglass (i.e., FR-4), ceramic, polyimide, flex, capton, copper-clad invar, etc.
2. Description of Related Art
Components of the above-noted type are generally classified as being of a surface mount or through hole mount type. Surface mount components are attached on the surface of a substrate to electrically conductive lands disposed thereon, principally by solder. Through hole mount components are, principally, attached mechanically and by solder, via a component lead or leads passing through (plated or unplated) holes in the substrate. Various adhesives may, additionally, be employed in the attachment of components with respect to a substrate.
Examples of surface mount components include (but are not limited to) leaded components, such as PLCCs, flatpacks, SOTs, PQFPs, sockets, connectors and the like; and leadless components such as capacitors, diodes, LCCCs and the like. Through hole components are generally of the leaded variety and include such devices as DIPs (Dual In-line Packs), axial-leaded resistors, transistors and the like.
Modern electronics are incorporating an everincreasing percentage of surface mount components in high density assemblies. These electronic assemblies are often very expensive and sensitive to thermal and/or mechanical damage, electrostatic discharge (ESD) and electrical overstress (EOS) damage. Products incorporating such assemblies include fax machines, pagers, personal computers, communication systems and a variety of other consumer and industrial goods. This has brought about the need for a safe, economical, rapid and effective means (i.e. proper means) by which such components can be removed and installed with respect to the substrate, either during the initial assembly and rework of the electronic assembly (i.e., circuit board), or during repair in the field.
A number of devices have been developed to deal with this task. The assignee of the present invention holds U.S. Pat. Nos. 4,605,152; 4,620,659; and 4,899,920, for example, which disclose large bench-top devices for the installation and removal of surface mount components (SMCs) utilizing convective (heated air or gas) means to effect solder reflow attendant to such installation and removal procedures. Such devices are effective, but are relatively costly.
Conductive (i.e, by contact) heating devices are less costly but, to be effective, must have sufficient thermal capacity and make good physical contact with all of the solder joints, leads or lands in order to properly transfer heat and effect solder reflow. U.S. Pat. Nos. 3,804,320 and 4,034,202, have addressed the installation and removal of the, then, emerging technology of multi-leaded integrated circuits (i.e., DIPs) and other through hole components via a manual, conductive method which employed tweezer-like handles and heated tips to effect mechanical gripping, reflow and removal of through hole components. Other similar devices have followed, yet there were many problems associated with all of them.
Proper removal of a surface mount component, especially a large thermally massive component on a high thermal mass substrate (i.e. a multilayer substrate or one with a large ground or voltage plane), requires that a sufficient amount of heat be delivered to all of the solder joints simultaneously in a rapid yet controlled fashion in order to effect complete, even solder reflow. The one step removal of a through hole component, such as a DIP, presents similar requirements, yet is not quite as difficult as the proper removal of a four-sided SMC, as will be revealed in more detail below.
The existing tweezer-type heating devices similar to those described in U.S. Pat. Nos. 3,804,320 and 4,034,202 (sold by Nu-Concept Systems, Inc., assignee of these patents) and other known tweezer-type heating devices (i.e, the HOT GRIPS.RTM. of Plato Products, Glendora, Calif. and the SMD soldering tweezers of Knurr AG, Munich, Germany) cannot effect the proper removal of a great many through hole and surface mount components, due to inherent deficiencies in their design. These design deficiencies can be classified into two important requirement areas, namely, thermal throughput and thermo/mechanical contact.
Thermal throughput refers to the device's ability to produce and deliver heat--in sufficient quantity and at a sufficient rate--to the tips which, in turn, transfer heat to the solder joints via contact, typically with the component lead and/or the solder itself, and effect solder reflow. This is determined by the physical layout of device; the wattage, thermal coupling of the heater, bobbin, and tip with respect to one another; available power to the handpiece; and other factors; and is generally manifested in the ability of the device to (1) maintain the tips at a sufficiently high idle temperature, (2) continually drive heat into the tips and, in turn, the joints during the joint heating phase, and (3) quickly return the tips to their preselected idle temperature after being depleted of heat from having contacted the work (i.e, tip temperature recovery). Tips for the larger surface mount components (i.e, 84 lead PLCCs or 132 lead PQFPs and larger) are rather massive and have relatively large surface areas. Such tips lose a great deal of heat through dissipation and thus, require a great deal of thermal energy to remain at a sufficiently high temperature to effect proper solder reflow. This temperature should typically be between 400.degree. and 900.degree. F. depending on the particular application.
However, existing heated tweezer-type devices of the type described above lack the power and/or thermal efficiency to maintain tips for large surface mount components at sufficiently high temperatures. Such devices also lack the thermal throughput required to continually supply a sufficient amount of heat during the joint heating phase, so that complete solder reflow is achieved in a safe and acceptably rapid time period. In many cases, SMC removal cannot be achieved at all because complete solder reflow cannot be achieved. Finally, these devices lack the thermal throughput to achieve rapid idle tip temperature recovery, so that successive SMC removal operations cannot be successfully accomplished in an acceptably rapid fashion.
Thermo/mechanical contact refers to the ability of the tip/handpiece combination to make reliable mechanical contact with all of the solder joints, simultaneously, so that heat is rapidly transferred into the joints in order to effect the rapid, complete, simultaneous solder reflow that is required for proper component removal. Proper thermo/mechanical contact is achieved when the operator can successfully accomplish this with relative ease. The design elements of these devices which affect thermo/mechanical contact are: the design and configuration of the handles, the hinge, alignment of heaters, alignment of tips, orientation and engagement of the tips to the heater, precision of the tips, travel of the handles/tips during use, and other important factors such as the ability of the operator to achieve good thermo/mechanical contact using the device.
Prior devices, such as those described above, lack the ability to achieve proper thermo/mechanical contact for many components, especially larger SMCs. The Nu-Concept devices, described in U.S. Pat. Nos. 3,804,320 and 4,034,202, were designed primarily to remove DIPS in a one-step operation. In order to effect proper thermo/mechanical contact with the leads of a DIP, the surfaces of the opposing tips have to engage the parallel opposing rows of leads in a parallel fashion in order to make good thermo/mechanical contact or "thermal engagement". Due to inherent tolerances in the device, the tips (including their engagement to each other) and in the component itself, parallelism of the tips with respect to the component leads (as well as with respect to each other) is often difficult, if not impossible to achieve.
U.S. Pat. Nos. 3,804,320 and 4,034,202 addressed this problem by providing a hinge pin tolerance or movement, causing the tips and the device's legs to pivot slightly (along a "y" axis that is parallel to the length of the legs of the tweezer and normal to an "x" axis which passes through the hinge and about which the legs of the tweezer articulate "open" and "closed" with respect to each other ), so that any deviation from parallelism could be overcome by squeezing the handles tightly enough to shift the tip into a parallel relationship to the opposing parallel rows of leads on a DIP. However, this solution does not work very well since it necessitates a very hard squeezing action on the part of the operator (which could damage the component as well as the device) and the coincidence of the "y axis" tolerance in the hinge with the error in parallelism of the tips, which often does not occur.
Another problem associated with the device described in U.S. Pat. No. 3,804,320, is that varying widths of DIPs or other devices could cause the planar surfaces of the tips to not engage the plane formed by the vertical portions of the leads of the DIP in a parallel fashion. A similar problem would exist with SMCs. Such parallel contact of the planes is desirable to maximize the contact area or thermal linkage for optimal heat transfer and solder joint reflow. Although this lack of planar parallelism is noted as intentional and desirable in U.S. Pat. No. 4,034,202 for the purpose of minimizing the transfer of heat to the component body, it is generally desirable to have maximum thermal contact area with leads of SMCs (especially large SMCs) due to the generally greater thermal mass of such components and correspondingly greater heat required for complete solder reflow.
This problem arises due to the inherent design of these devices. Since the tips move along an arc as they are brought together, their angularity must take into account the width of the component if the tips are to engage the component leads in a perfectly planar or near perfectly planar fashion. Although this problem can be addressed by employing dedicated (as opposed to interchangeable) handpiece/tip combinations as may be suggested by U.S. Pat. No. 4,034,202, this would require having a very large number of handpieces, cords and power sources to conveniently deal with today's everincreasing diversity of SMC sizes and configurations in practical environments.
Prior devices discussed above also did not anticipate the even greater challenges of establishing proper thermo/mechanical contact with four-sided SMCs. The additional requirements of four-sided devices over two-sided devices include the precision of the angle (typically 90.degree.) between adjacent sides of the tip portion on each leg of the tweezer handpiece. Additionally, it is even more important with four-sided SMCs that the bottom edges of the two tip halves lie in a common plane, which, is generally (but not always) perpendicular to the longitudinal or y-axes of the tweezer legs. Manufacturing of such tips to a sufficient degree of precision can be very expensive in that secondary machining processes are required.
Prior tweezer-like heating devices employ a relatively long arcuate travel distance against a spring load until the tips contact the component. This can be very inconvenient and uncomfortable to the operator. In addition, this travel distance makes it difficult to safely access a component in a high-density electronic assembly in which adjacent components can typically be 0.050 in. away or closer.